Myosin is found in all eukaryotic cells and appears to be involved in diverse cellular motile processes such as cytokinesis. There are at least two genes for vertebrate nonmuscle myosin heavy chains (MHCs), MHC-A and B. The nonmuscle MHC-A gene is expressed abundantly in fibroblasts, epithelial cells and lymphoid cells, but less abundantly in neuronal cells and differentiated muscle cells. We have been studying the mechanisms responsible for the cell type-dependent transcription for the nonmuscle MHC-A gene. We have previously identified three clustered cis-regulatory elements (A, C and F) in intron 1, which modulates transcription in a cell type-dependent manner (J. Biol. Chem. 273, 9168-9178, 1998). One of these elements (element F) contains an E-box sequence. In this study, we undertook to identify trans-acting factors responsible for element F-mediated regulation. Yeast one hybrid screening identified TFEC-1, TFE3 and USF2, all of which contain basic-helix-loop-helix and leucine zipper motifs, as element F-binding proteins. We then analyzed the binding activities of these proteins as well as TFEC-s and USF1 by mobility shift assays using in vitro translated full-length proteins. All five proteins bound specifically to element F with the following relative binding affinities: USF1=USF2 greater than TFEC-1=TFEC-s greater than TFE3. To investigate the trans-acting activities of these proteins, eukaryotic expression constructs were cotransfected into HeLa and NIH3T3 cells with luciferase reporter constructs which consist of the core promoter and element F of the MHC-A gene. All five proteins were found to activate reporter gene expression in an element F-dependent manner. TFEC-1 and TFE3 show the highest activities, whereas USF1 and 2 show the lowest. These results suggest that TFEC, TFE3, USF1 and 2 can activate transcription of the nonmuscle MHC-A gene via element F. - transcription; nonmuscle myosin heavy chain-A gene; tissue-dependent expression